How to properly parse a string?

Question

PHP

Good afternoon!
I have variables containing strings, some of which contain an element consisting of 2-3 numbers and one letter.
For example:

$a = 'Service code 145a is used for...';
$b = '...you are to use code 99a.';

I need to somehow parse the values ​​of a variable that may contain similar code. The number and the digits themselves are unknown, but it is known that there is always an "a" after 2-3 digits. The position of this code in the line can change.
How can I make a template, for example for preg_match, that allows you to find such codes in strings?
Ultimately, I need to leave only the code.

Answer 1

/(\d+)a/

Answer 2

https://regex101.com/r/5A4mDU/1/